Salinity quickened Sundari decrease

Sundari, the iconic mangrove species responsible for naming the entire area as Sundarban, is disappearing fast in both Indian and Bangladesh Sundarban mainly due to climatic impacts and range of anthropogenic reasons.
A 2015-16 survey report by Kolkata based nonprofit Nature Environment Wildlife Society (NEWS) has found that the Indian Sundarban has lost 90 per cent of its Sundari trees in last five decades. The situation in Bangladesh Sundarban is no different. Another report prepared by IUCN in 2017 corroborates the NEWS finding which pointed out that Sundari has been declining both in pure as well in mixed growth of mangroves in Indian Sundarban.
According to two different studies conducted by Dhaka University and Jahangir Nagar University respectively, the population of Sundari has also been gradually decreasing in Bangladesh Sundarban. “There has been 10 per cent decrease in number of Sundari trees in Khulna range from 1990 to 2010” pointed out one study. Khulna range incidentally holds significant part of the Sundarban forest area in Bangladesh.
Biswajit Roychowdhury, a wild life expert and member of West Bengal wild life advisory board, said to Sundarban Beyond Border that Sundari population has now got mostly restricted in the core forest area of Chamta within Sundarban Tiger Reserve and along the heavily forested bank of river Raymangal of West Bengal Sundarban; based on the recent study carried out by nonprofit NEWS.

From boat building to saline surge

According to experts while historically, during British era and even in free India, Sundari was mercilessly felled to manufacture boats or furniture; the recent decline of Sundari in transboundary Sundarban can be attributed to increasing salinity; triggered by both changing climate and anthropogenic interferences. The forest area, crisscrossed by numerous rivers and rivulets being constantly fed by fresh water from upper stream through rivers like Ganges; Ichamati and others, is turning increasingly saline due to combination of larger ingress of sea water and more evaporation due to increase of sea surface temperature – both being triggered by changing climate – as well as reduction in freshwater supply from upstream due to several man made hindrances. Sundari along with Keora are the worst sufferers as they are known to be less salt tolerant among various mangrove species found in Sundarban.

The Bangladeshi research paper states that increase in salinity has been seriously affecting the Sundari trees by decreasing its population at an annual rate of 3 to 9 per cent. Another research communication, ‘Aquatic Salinization and Mangrove Species in a Changing Climate’ published in 2018 by World Bank, also admits huge impact on the mangrove ecosystem due to salinity rise in rivers flowing adjacent to transboundary Sundarban.

According to experts, apart from increase in salinity within river water; enhanced alkalinity in soil, river pollution and felling of trees are also responsible for diminishing Sundari population in Sundarban. The IUCN report says that the average height of the Sundari trees has been gradually decreasing over the decades and the trees have been affected by a disease called top-dying diseases in which the trees start to die slowly from its upper portion and finally collapses on the ground. IUCN report also points out that “approximately 70 per cent of H. fomes (Sundari) stems have been estimated to be moderately or severely affected by the top – dying disease, which causes high mortality, threatening mangrove diversity , forest cover and ecosystem complexity”. Roy Choudhury also complaints that top – dying disease is observed in Indian part of Sundarban as well along with as narrowing of the girth of the trees.

Overall mangroves at risk

‘State of Art Report on Biodiversity in Indian Sundarban’ by WWF-India, also points out that climate change impact alongside industrial pollution, destruction of mangrove habitat, growing human population putting pressure on the forest, exploitation of forest resources and absence of adequate high land inside the forest are threatening the growth of mangroves in Sundarban, particularly Sundari.

Indian state of forest report 2017 shows that overall mangroves are in crisis in one of its biggest global addresses as it points out that that where entire country has witnessed 181 sq km increase in mangrove habitation over the period 2015 to 2017, during the same period mangrove population within Indian Sundarban grew merely by 8 sq km though the area holds 43 per cent of country’s mangrove population. According to sources, another four mangrove species alongside Sundari are also in the IUCN red list; which are Keora, Goran, Bayen and Dhundul.

Mangroves vs salinity 
• Low tolerance : Heritiera (Sundari) and Sonneratia ( Keora) species
• Medium tolerance Phoenix (Hental), Excoecaria (genwa), Bruguiera (Kankra), Xylocarpus(Dhundul), Aegiceras (Kholsi), and Rhizophora (Garjan) species
• High salinity Avicennia (Bayen) and Ceriops (Goran) species